Surfactants exhibit a phenomenon called the critical micelle concentration (CMC) where the concentration of a surfactant reaches a point, in which the surfactant molecules aggregate to form micelles. Micelles are heterogeneous spherical formations which have a polar exterior and non-polar interior. Once the micelles form, they exist in a dynamic equilibrium with the bulk surfactant solution. All surfactants exhibit a unique CMC based on their structure, chemical properties, and environmental conditions. It has been shown that β-diketones can be used as probe molecules to determine the CMC of surfactants. The aim of this project is to use novel β-diketones as probe molecules to determine the CMC of different surfactants using UV-Vis spectroscopy. Ketones are one of many functional groups that form structural isomers through tautomerism. In the case of β-diketones, two structural isomers are formed: the keto and the enol isomers, which exist in equilibrium. The abundance of each tautomer, in connection to the surfactant concentration, can be determined because when micelles form, the enol isomer of the diketone can stabilize itself within the non-polar interior of the micelles. UV-Vis spectra at different surfactant concentrations show that the absorbance associated with the keto and enol isomers can change drastically upon reaching the CMC. For example, when benzoyl acetone is used as a probe, we have observed that the enol absorbance increases while the absorbance of the keto decreases, following micelle formation. By monitoring both absorbance changes, the concentration in which micelles form can be determined. An additional goal of this project is to use green solvent systems as environmentally friendly alternatives to modify the surrounding solution of our systems and determine how that could affect a β-diketone’s tautomerism ratio as well as the CMC of different surfactants.

Systematic studies of critical micelle concentration of surfactants using novel β-diketone relationships and green solvents

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